Known technique in the art is depicted e.g. in the articles “Roasting developments—especially oxygenated roasting, Developments in Mineral Processing, Volume 15, 2005, Pages 403-432 K. G. Thomas, A. P. Cole”, “Roasting of gold ore in the circulating fluidized-bed technology Developments in Mineral Processing, Volume 15, 2005, Pages 433-453, J. Hammerschmidt, J. Guntner, B. Kerstiens”, and patent documents WO2010/003693, U.S. Pat. No. 6,482,373, AU 650783, U.S. Pat. No. 4,919,715.
Copper and gold concentrates containing arsenic are preferably processed by dearsenifying roasting before further treatment by smelting in a copper plant or cyanide leaching.
The dearsenifying roasting is made by controlling the oxygen potential during arsenic volatilization to maintain iron both as magnetite and pyrrhotite. The calcine is thereafter further processed by conventional matte smelting in case the raw material consists of a copper containing concentrate. Gold rich calcine are often processed by cyanide leaching but the leaching is only efficient, if the calcine is dead roasted or sulphating roasted. A conventional method to treat arsenic rich gold concentrates is therefore a two stage roasting process whereby both stages consist of fluidized beds.
The first fluidized bed is the dearsenifying step, operating at a very low oxygen potential, and the second fluidized bed is the dead roasting or sulphating step, operating with an excess of oxygen.
The process gas leaving the dearsenifying roasting will contain sulphur rich gas compounds such as elementary sulphur, hydrogen sulphide and arsenic sulphide while the process gas leaving the second oxidizing roasting will contain oxygen and oxidized compounds like SO3.
The roasting process gas is normally further processed by separation of calcine and process gas in cyclones, post combustion, gas cooling and dust cleaning in electrostatic precipitator and possibly bag filter and finally conversion of SO2 to sulphuric acid.
Known problems in the further processing are:                The forming of accretions which could fall down and damage equipment or block gas passage. The accretions are e.g. formed due to local under cooling of the process gas or on cold surfaces in the equipment.        Condensation of arsenic on cold surfaces which will form said accretions.        Condensation of acid mist on cold surfaces which will cause corrosion and contribute to formation of accretion.        If the acid mist quantity is high, the cost for effluent treatment will also be high.        Heat recovery of the system has often been limited to the production of saturated steam which is less favourable for the production of electric energy.        
These problems were solved before in the following ways:
The post combustion air can for instance be added at the cyclone exit. However, in some cases the post combustion can cause accretions in gas ducts and this is more likely if the combustion is made with a large amount of air in ambient temperature.
The forming of accretions on cold surfaces in the equipment is normally solved by the use of preheated air, which requires separate heating equipment with increased investment cost and operating costs (maintenance and possibly heating fuel). Forming of accretions in the equipment is normally avoided by insulating the equipment well so that no cold surfaces exist, although it is also accepted that accretions will form where the insulation is damaged or not properly done.
Gas cooling during two-stage roasting can either be made by direct cooling with water injection in a cooling tower or through indirect cooling through cooling coils in the fluidized beds and by conventional steam boiler. Lead and arsenic contents in the concentrate and SO3 concentration in process gas influence suitable cooling method, since these elements can cause formation of accretions on cooling surfaces.
Example of compounds that forms accretions are elemental lead at cooling coils in the first dearsenifying stage and SO3 or arsenic trioxide at the boiler tubes of the steam boiler. It is today generally accepted that high SO3 concentrations cause higher cost in the effluent treatment plant.
Corrosion of the equipment is normally avoided by insulating the equipment well so that no or little SO3 condensation occurs, although it is also accepted that corrosion will occur over time and for example where the insulation is damaged or not properly done. It would be best to be able to avoid high SO3 concentrations in the process gas. This is today, to some extent, done by controlling the process with modern control systems. Further reductions would be an advantage.
Heat recovery in form of steam is today done by steam generation coils in the fluidised bed itself often without any superheating. A normal steam boiler is sometimes used in the process gas stream, but with similar risks of accretions forming and corrosion as described.